1. Field of the Invention
The present invention relates to processes for producing polypropylene resin compositions. More particularly, it relates to processes for producing polypropylene resin compositions in which no or substantially no fish eyes develop and from which molded articles with good appearance can be produced.
2. Description of the Related Art
Polypropylene is used in a wide variety of fields such as automotive applications, electric/electronic applications and packaging applications because of the fact that it is inexpensive and it is superior in mechanical characteristics, moldability, heat resistance, solvent resistance and appearance.
For applications which require impact resistance, propylene-ethylene block copolymers containing ethylene-propylene copolymers are used. Particularly in injection molding applications, from the view points of fluidity, impact resistance and molded articles' appearance, propylene-ethylene block copolymers are used that contain a polymer portion which is composed mainly of propylene and have a relatively low molecular weight and an ethylene-propylene copolymer portion which has a relatively high molecular weight, in other words, has a high intrinsic viscosity.
However, when a propylene-ethylene block copolymer which contains an ethylene-propylene copolymer portion having a high intrinsic viscosity is produced by continuous polymerization, the ethylene-propylene copolymer portion disperses insufficiently and, as a result, fish eyes develop. Fish eyes will impair the appearance or reduce the impact resistance of molded articles.
JP 11-124439 A discloses, as a method for producing a polyolefin having less fish eyes, a technique which includes controlling the temperature and shear rate applied to a resin by means of a kneading machine when melt-kneading and pelletizing the resin.
JP 2001-150429 A discloses, as a method for inhibiting the development of fish eyes in polyolefin, a method which includes controlling separately two independently-provided devices, namely, a device for melting and a device for kneading.
It is possible to reduce fish eyes to some extent by the prior art techniques, but those techniques have not exert satisfactory effects yet and there is a demand for more improvements.